The present invention relates to the field of prostheses and, more particularly, to a prosthesis and a method for treating ailments of the scaphoid (os scaphoideum).
The scaphoid is one of the eight or nine principal bones forming part of the wrist in humans. The wrist is a very complicated joint since it consists, not of two bones rubbing against one another, but of eight or nine principal bones of unusual shapes held and moving in equilibrium under control of a highly developed system of ligaments. The scaphoid is of particular importance clinically because it is the wrist bone which tends most often to be fractured.
The scaphoid is the largest of the bones located in the first row of wrist bones (known as the first carpal row or proximal carpal row). The other principal bones in the proximal carpal row are the lunate, triangular and pisiform bones. The bones of the proximal row are articulated to the radius (of the forearm) and the articular disk. The second carpal row (also known as distal row) contains the trapezium or greater multangular, the trapezoid or lesser multangular, the capitate and the hamate. The bones of this second carpal row are firmly attached to the metacarpal bones of the hand.
The scaphoid is surrounded by the trapezium, trapezoid, capitate, lunate and radius, as illustrated in FIG. 1 (which illustrates the bone of the right hand viewed looking towards the palm). The scaphoid is "articulated" at the proximal side with the radius and the distal side with the trapezium and trapezoid. Fractures of the scaphoid tend to occur, in around 70% of cases, in the central third thereof, as illustrated by the shaded area F in FIG. 1. If the fracture is not properly treated then a pseudathrosis or necrosis of the proximal bone fragment can occur. This is because, in a third of cases, blood supply to the scaphoid is furnished only by vessels at the distal side. Other ailments too, besides fractures, can lead to damage to or degeneration of the scaphoid.
It could be contemplated to treat ailments of the scaphoid by implanting a prosthesis into the wrist of a patient. However, in order for such a technique to be successful, the prosthesis in question must allow the patient to retain or regain mobility of the wrist joint. Further, the prosthesis must be stable, that is, the prosthesis and the bones of the wrist must correctly return to their original positions after a movement has been terminated, without dislocation. Other important considerations are the comfort of the implant for the patient, at rest and during different types of movement and the biocompatibility and durability of the prosthesis.
It is not straightforward to determine the appropriate combination of shape, size and material enabling a suitable prosthesis to be produced for use in treating ailments of the scaphoid. For example, the present inventor has found that there are disadvantages involved in the use of certain of the materials which might have been contemplated, notably polyethylene, ceramic zircon and titanium.
In the case of polyethylene, the prosthesis is too soft, i.e. its modulus of elasticity, also called Young's modulus, which is of the order of 1 GPa (Giga Pascal), is far too low compared with that of bone, the latter being comprised within the range of 15 to 25 GPa. Such a prosthesis is therefore subject to deformation and becomes crushed after a relatively short period of use. Wear debris resulting from deterioration of the prosthesis can then migrate in some areas of the patient's wrist, which can cause painful inflammatory reactions for the patient.
In the case of zircon titanium, the prosthesis is too hard. Indeed, the respective Young moduli of zircon and titanium are respectively about 300 and 110 GPa, which is far too high with respect to the Young's modulus of bone. The prosthesis does not deform enough upon motions of the wrist, causing the bones in contact with the prosthesis to be stressed by the latter. Because of bad distribution of stresses, the patient experiences discomfort, for example, when pressing a fist down onto a surface. Also, there is a significant risk of wearing out the cartilage or bony surfaces in contact with the prosthesis due to the hardness of the prosthesis material.